The invention relates generally to a system for producing a single crystal material in which impurities, dopants, and oxygen are well controlled and substantially uniformly distributed, both axially and radially.
Semiconductor materials are used extensively in the electronics industry for the fabrication of devices such as integrated circuits, diodes, transistors and photovoltaic cells.
While semiconductor materials such as gallium arsenide are important for many applications, particularly as substrates for exceptionally fast devices useful in many military applications, presently, silicon is the most widely used semiconductor material. In particular, silicon is used as the substrate for most integrated circuit devices.
One well known characteristic of these materials is the sensitivity of their electrical properties to factors such as impurities and crystal defects. For example, the addition of less than 0.01 weight percent of a particular type of impurity can increase the electrical conductivity of a typical semiconductor like silicon or germanium by six or seven orders of magnitude. In any material, there will be some percentage of impurities that are present in the source material or which enter the material during its manufacture and subsequent processing, such as oxygen. In addition to impurities that are present in the source material or result from the manufacturing process, it is often desirable to dope a semiconductor material by selectively introducing impurities that are incorporated into the crystalline structure. This incorporation is a function of the segregation coefficient which is unique to that particular element.
As is well known, boules of silicon are typically grown using the Czochralski process by withdrawing a seed crystal from a crucible of molten silicon. The boule is sliced horizontally to form thin circular "wafers", which are then overlaid with silicon dioxide or other insulating materials, metal conductors, and other semiconductor materials. Typically, the wafers have a diameter of up to about four inches and are divided into a number of individual integrated circuits.
Because the operation of a semiconductor device depends on the electrical properties of the material forming the device, it is highly desirable to use silicon that has a well controlled, uniform concentration of oxygen and dopants. Minimizing variations in quantity and distribution of impurities is even more crucial in the production of very large scale integrated circuits (VLSI's) or to fabricate one large circuit, such as an entire computer, on a single wafer to reduce the length of connections and increase the overall speed of operation of the computer.
At the outset, it is important to note that liquid silicon is a difficult material to handle. It is liquid only at extremely high temperatures, in excess of 1400.degree. C., it is opaque, and it has unusual surface tension and wetting properties which cause it to behave more like mercury than water. In addition, a growing single crystal can readily be contaminated by carbon and other elements or become polycrystalline due to contaminants or changes in parameters at the growth zone such as the temperature profile.